Since we do not know the time taken to row upstream and downstream separately, let " " be the time taken to row upstream and let " " be the time taken to row downstream. Let "v" be their speed in still water.

Then their speed upstream is and that equals "distance divided by time: .

Their speed downstream is and so .

Those, together with give three equations to solve for the three values , , and v. Of course, you are only asked for v so if you could find a way to eliminate and , reducing to a single equation in v, that would be excellent.